Circuit arrangement for the analogue suppression of echos

ABSTRACT

A circuit arrangement for the analogue suppression of echoes, as in particular can be used in a hybrid-circuit for DSL-transmission systems, comprises a replica ( 8 ) for emulating the behaviour of the transmission line ( 17 ). In addition, a circuit ( 3, 4 ) for emulating the behaviour of the transmitter ( 13 ) is provided, which comprises at least one lowpass ( 3, 4 ). Furthermore, a replica ( 9, 10 ) for emulating the behaviour of bridge taps ( 14 ) can also be provided, which comprises at least one bandpass ( 9, 10 ). Additionally, a replica ( 19 ) for emulating the behaviour of the line driver ( 1 ) can also be provided.

[0001] The present invention relates to a circuit arrangement, which isprovided for the analogue suppression of echoes as regards the receivedsignal of a communication device connected via a transformer to atransmission line.

[0002] In the case of ISDN and DSL transmission systems, data istransmitted between the exchange and the subscriber via a twisted linewire pair, whereby each of the two line wires is provided simultaneouslyfor both transmission directions, that is to say, for the transmissiondirection from the exchange to the subscriber and for the transmissiondirection from the subscriber to the exchange. The signal lying on theline wires is, therefore, composed of a received signal portion and atransmitted signal portion. In order to obtain the received signal—alsodescribed as “far end signal”—on the subscriber end, the transmitted orecho signal—also described as “near end echo”—must be subtracted fromthe overall signal. On the one hand, this can be achieved by adaptiveecho canceling, which is implemented digitally. On the other hand, thiscan be achieved by a so-called hybrid-circuit, which simultaneouslyexecutes two-wire/four-wire conversion, in order to separate thetransmitted and received signal. Hybrid-circuits are, for example, usedin telecommunication transmitting apparatus. Possible configurations ofknown hybrid-circuits are, for example, described in “Telephone VoiceTransmission”, Winston D. Gayler, Prentice Hall, 1989.

[0003] Echo suppression by a hybrid-circuit has two importantadvantages. On the one hand, the ratio between the received signal (“farend signal”) and the transmitted or echo signal (“near end echo”) isboosted by the degree of echo suppression, typically by approx. 20 dB.As a result, less strict requirements on the signal to noise ratio ofthe analogue/digital converter provided for further processing of thereceived signal are necessary. On the other hand, in conventional ISDN-or xDSL-transmission systems, a digital linear echo compensator iscoupled after the analogue part of the received signal path. Thisdigital linear echo compensator is not able to compensate non-lineardistortions of the transmitted signal path, which are found in the echo.Therefore, the ratio between the non-linear distortions and the actualsignal portion in the received signal path must be kept very low. Theecho suppression of a hybrid-circuit can support this, since the portionof the transmitted signal in the received signal and, therefore, thenon-linear distortions, are reduced.

[0004] The main technical problem, which arises in the case ofhybrid-circuits, is the precise emulation of the transmitted signalrelated to that point in the transmission system, at which the emulatedtransmitted signal should be subtracted from the overall signal. Thetransmitted voltage occurring at a particular point of the transmissionsystem is, essentially, a function of the impedance valid at this pointof the transmission line as well as of the transformer provided fortransmission. This impedance is greatly variable, since lines ofdifferent material and length are used in the application with orwithout so-called “bridge taps”. The term “bridge tap” describes stubcables connected to the line wires of the transmission line, which areprovided for the connection of further subscribers, but which are notterminated with a suitable intrinsic impedance and can cause reflectionsas a result. The problem, therefore, arises of emulating as accuratelyas possible the impedance of the transmission line and transformer inall cases of application. In the case of an SDSL transmission system(“symmetric digital subscriber line”), such impedance simulation must beoptimised in the frequency band of 0 to 400 kHz.

[0005] A further system-dependent requirement on the hybrid-circuit inan SDSL transmission system is a signal to noise ratio of at least 90dB. Further, the power loss of the hybrid-circuit should be as low aspossible.

[0006] Various circuit arrangements for the suppression of analogueechoes are known from the state of the art.

[0007] Thus, for example, in “A CMOS analogue front-end IC for DMTADSL”, C. Conroy et al., 1999 IEEE International Solid-State CircuitsConference, ISSCC 99, Session 14, Paper TP 14.2, it is proposed toimplement two identical transmitted signal paths, whereby the firsttransmitted signal path is used for the primary transmitted signal andthe second transmitted signal path to emulate the echo voltage, in orderto be able to supply these subsequently on systems with analogue echosuppression.

[0008] In “An Integrated Adaptive Analogue Balancing Hybrid for Use inADSL Modems”, F. Pécourt et al., 1999 IEEE International Solid-StateCircuits Conference, ISSCC 99, Session 14, Paper TP 14, it is proposedto emulate the transmitted signal appearing in each case at theinteresting point of the transmission line as a result of the fact thatthe transmitted signal produced by the line driver of the particularcommunication device is filtered by means of an integrated activefilter. The filter produces an emulation of the echo occurring in thereceived signal path, so that echo suppression as regards the receivedsignal can be achieved through subsequent subtraction of the outputsignal of the filter from the received signal of the correspondinghybrid-circuit.

[0009] Finally, a generic circuit arrangement for the suppression ofanalogue echoes is known from “A 25kft 768Kb/s CMOS Transceiver forMultiple Bit-Rate DSL”, M. Moyal et al., 1999 IEEE InternationalSolid-State Circuits Conference, ISSCC 99, Session 14, Paper TP 14.4. Inthis document, a hybrid-circuit is proposed, in which the transmissionline is emulated by a scaled impedance model, a so-called replica. Partsof this replica are constructed outside the chip. Additionally, in thisdocument, it is proposed to emulate the main and scatter inductance ofthe transformer inside the chip by scaled inductive resistors, wherebythese inductive resistors are implemented by so-called gyrators. Ingeneral, gyrators are understood to mean active circuits with, forexample, operation amplifiers and capacitors, which simulate thesewithout using an inductive resistor. Bridge taps are emulated by aRLC-network (“replica”) inside the chip, whereby the inductive resistoris also implemented here by a gyrator circuit.

[0010] The previously described solutions for emulating the transmittedsignal, related to that point in the transmission system, at whichsubtraction from the received signal should take place, have variousdisadvantages. In the case of the first solution described above,considerable technological circuit complexity is necessary, since theoverall transmitted signal path must be implemented twice. Any mismatchbetween the two transmitted signal paths results in inadequate echosuppression. In the case of the second solution previously described,the active filter provided to filter the transmitted signal is activeover the overall frequency band. Considerable power consumption isrequired to keep noise within tolerable limits. In the case of the thirdsolution previously described, on the other hand, the gyrators areactive over the overall frequency band. Very high power consumption isalso required in this case to keep noise within tolerable limits.

[0011] The present invention is, therefore, based on the objective ofproposing a circuit arrangement for the suppression of analogue echoes,in particular for combination with a hybrid-circuit, in which thepreviously described disadvantages do not arise and satisfactoryanalogue echo suppression can be achieved at minimum cost.

[0012] This object is achieved, according to the invention, by a circuitarrangement with the features of claim 1. The subclaims in each casedefine preferred and advantageous embodiments of the present invention.

[0013] The circuit arrangement according to the invention comprisesfirst circuit means for emulating the behaviour of the transmissionline, whereby these first circuit means, in particular, can beimplemented by a passive RC network. Further, the circuit arrangementaccording to the invention comprises second circuit means for emulatingthe behaviour of the transformer, whereby these second circuit meanscomprise one or several lowpasses of any level, to which the transmittedsignal transmitted by the communication device, that is to say, theoutput signal of the corresponding line driver, or a correspondingsignal of the first circuit device are supplied directly as inputsignal. Furthermore, the circuit arrangement according to the inventionhas third circuit means, which, in particular, can be designed in theform of an adder, in order to correct a signal picked up on thetransformer (overall signal), which is composed of a received signalportion and a transmitted signal portion, by a signal of the firstcircuit means and a signal of the second circuit means and, therefore,to obtain the received signal with suppressed echo.

[0014] The behaviour of any bridge taps provided in the transmissionline can be emulated by one or several bandpasses connected in parallel,to which either the transmitted signal directly or a correspondingsignal of the first circuit means is supplied as input signal. Thebandpasses just as the lowpasses are connected on the output side to theadder.

[0015] It is especially advantageous for emulating the behaviour of thetransformer if two lowpasses connected in parallel are used, whereby theone lowpass is a lowpass of first order and the other lowpass a lowpassof second order. All the voltages or signals supplied to the adder, thatis to say, in particular, the output signals of the low and bandpasses,can be weighted with corresponding factors, that is to say, withcorresponding real positive or negative figures, whereby, in particular,if two lowpasses connected in parallel are used, emulation of thebehaviour of the transformer is advantageous, if, for the weightingfactors c1 and c2 of the two lowpasses, the following equation applies:c2=1−c1.

[0016] In order to match the circuit arrangement according to theinvention adaptively to the particular transmission line used in eachcase, not only the weighting factors are advantageously variablyformulated, but operating parameters of the low and bandpasses, as, forexample, the limit frequencies of the low and bandpasses and the qualityof the bandpasses, can be adaptively matched directly to thetransmission line.

[0017] When building up the communication over the transmission line,the optimum parameters of the low and bandpasses are determinedadaptively by the digital part of the chip used in each case. Anadvantageous implementation of the circuit is to provide the variablelow and bandpasses as well as the adder inside an integrated circuit,but to construct the first circuit means, that is to say, the replica,for emulating the behaviour of the transmission line, externally. Thishas various advantages. On the one hand, the low and bandpasses can beadjusted adaptively by an equally integrated digital part to thetransmission line, without requiring pins for the control signals. Onthe other hand, the low and bandpasses as well as the adder can besatisfactorily implemented by integrated operational amplificationcircuits. In addition, the resistors of the replica of the transmissionline must be as small as possible in order to minimise noise. This makesrelatively large capacitors necessary, the integration of which would beuneconomic. A further reason for a relatively low-ohm replica of thetransmission line is the fact that the inputs of the low and bandpassesput stress on this replicator. Finally, an externally constructedreplica of the transmission line would also enable customers to choosethe optimum circuit device in each case for their particularapplication.

[0018] An important advantage of the present invention lies in the factthat the emulation of the echo related to that point in the transmissionsystem, at which subtraction from the overall signal should be executed,is as far as possible carried out with passive elements. These are highlinear and low noise. The low and bandpasses in each case are onlyeffective in a narrow frequency band and, therefore, influence linearityand noise to a minimum. Compared to the first known solution describedat the beginning according to the state of the art, the circuitarrangement according to the invention is considerably lesstechnologically complex, since the overall transmitted signal path doesnot have to be implemented a second time. Emulation of the echo signalis derived from the actual transmitted signal so that any mismatchbetween the two transmitted signal types would be insignificant. Sincethe low and bandpasses provided according to the invention in each caseare only effective in a narrow frequency band, considerably less powerconsumption can be achieved in comparison to the second and thirdsolutions known from the state of the art described above.

[0019] In particular, when using line drivers, which are based on theprinciple of a power source, so-called “current mode” line drivers, areplica of the line driver is used advantageously according to theinvention, whereby the output signal of this replica of the line driveror a signal picked up between the replica of the line driver and thereplica of the transmission line is supplied to the at least one lowpassas input signal. In this way, when using a “current mode” line driver,improved echo cancelling can also be achieved by the hybrid-circuit sothat the demands on the subsequent analogue/digital convertor arereduced.

[0020] The present invention is suitable, in particular, for thesuppression of analogue echoes in combination with a hybrid-circuit,such as in communication devices for ISDN and XDSL transmission systems,in particular, SDSL transmission systems or Gigabit Ethernettransmission systems etc.

[0021] The invention is explained below on the basis of preferredembodiments with reference to the accompanying drawing.

[0022]FIG. 1 shows the structure of a circuit arrangement for thesuppression of analogue echoes according to a first embodiment of thepresent invention,

[0023]FIG. 2 shows the structure of a circuit arrangement for thesuppression of analogue echoes according to a second embodiment of thepresent invention, and

[0024]FIG. 3 shows the structure of a circuit arrangement for thesuppression of analogue echoes according to a third embodiment of thepresent invention.

[0025]FIG. 1 shows a hybrid circuit of a communication device connectedto a transmission line 17, for example, an SDSL transmission line. Asnormal in itself, the communication device has a line driver 1, which isconnected via protective resistors R1 and R2 to a transformer 13. Thetransformer 13 represents the beginning of the transmission line 17.Stub cables or bridge taps 14 are connected to the transmission line 17.On the receiver end, a transformer 15 is equally provided, which isconnected on the input side to the transmission line 17 and isterminated on the output side by a terminal resistor 16.

[0026] As clear from FIG. 1, the output voltage of the line driver 1corresponding to the pure transmitted signal lies on circuit point Ashown in FIG. 1, whereas the sum of the received signal obtained overthe transmission line 17 and the transmitted signal to be transmittedover the transmission line 17 lies on line point B equally shown inFIG. 1. In order to obtain the pure received signal, therefore, thetransmitted signal portion, that is to say, the echo, must besubstracted from the voltage lying on circuit point B or the overallsignal. For this purpose, a voltage, which corresponds precisely aspossible to the echo voltage on point B, is produced in the waydescribed below.

[0027] On point A, a circuit (“replica”) of the transmitted signal pathis connected which emulates the behaviour of the transmitted signalpath. This replica comprises resistors R3 and R4 which represent areplica of the resistors R1 and R2, as well as an RC network 8, whichrepresents a replica of the transmission line 17. The dimensions of theRC network are such that the input impedance on point B corresponds tothe input impedance of a typical long transmission line without bridgetaps. The voltage occurring on point D corresponds in firstapproximation to the echo voltage lying on point B.

[0028] In order to keep the received signal with a greatly reduced echoportion on a circuit point C, the voltage lying on point B is suppliedto an adder which subtracts the voltage lying on point D from this,whereby both voltages can be weighted by devices 7 or 18 with factors dor a. With this method of operation, however, the behaviour of thetransformer 13 and the bridge taps 14 is not yet taken intoconsideration.

[0029] The transformer 13 with its main and scatter inductancerepresents a bandpass, the zero point of which is determined by the maininductance and the pole point of which is determined by the scatterinductance. Emulation of the main inductance is, therefore, possible bya highpass, whereby the behaviour of a highpass corresponds to that ofan inverted lowpass. In order to take into consideration, in the case ofthe circuit arrangement shown in FIG. 1, the behaviour of thetransformer 13, a lowpass 3 (LP1) is provided, to which the voltagelying on point D is supplied as input voltage, whereby the output signalof the lowpass 3 is supplied to the adder 2. The adder 2 subtracts fromthe voltage on point D the output voltage of the lowpass 3, so that onlythe differential voltage resulting from this is subtracted form thevoltage lying on point B. The circuit can be improved further by thefact that at least one other lowpass 4 (LPn) is connected in parallel tothe lowpass 3, the output voltage of which is likewise substracted bythe adder 2 from the voltage at point D. The outputs of the lowpasses3-4 are preferably weighted by means of devices 5-6 weighted withfactors c1-cn. Equally, limit or edge frequencies of the lowpasses 3, 4can preferably be adjusted so that the circuit can be matched to varioustypes of transformer.

[0030] Bridge taps 14 connected to the transmission line 17 in theimpedence progression over the frequency on point B cause local minimaand maxima, which depend on how far the open line end is away from theinput in relation to the wave length ([2·n+1]·λ/4=minima,[n+1]·λ2=maxima, n=0 . . . ∞). The bridge taps 14 can, therefore, beemulated by additive overlaying of the frequency responses from bandpassfilters with the signal on point D. In the case of the circuitarrangement shown in FIG. 1, therefore, bandpasses 9, 10, to which thevoltage lying on point D is supplied as input voltage, are provided. Theoutput voltages of the bandpasses 9, 10 are again supplied to the adder2, which subtracts these output voltages from the voltage picked up onpoint B. The circuit arrangement can be improved further, if the outputvoltages of the bandpasses 9-10 are weighted by corresponding devices11-12 with factors b1-bn. The bandpasses 9, 10 can preferably beadjusted in their quality, limit or edge frequency and amplification, inorder to be able to adapt these to the particular lines.

[0031] In the case of the circuit arrangement shown in FIG. 1, the inputvoltage of the low and bandpasses in each case correspond to the voltagepicked up on point D, which emulates the transmitted voltage of the linedriver 1, that is to say, the voltage lying on point A. Naturally, thetransmitted voltage of the line driver 1, that is to say, the voltagelying on point A, can also be supplied directly to the low andbandpasses as input voltage, whereby the low and bandpasses need nothave the same input signal.

[0032]FIG. 2 shows a hybrid circuit according to a second embodiment ofthe present invention, whereby the parts corresponding to the componentsshown in FIG. 1 are identified with the same reference numbers.

[0033] As clear from FIG. 2, the transmitted signal is not only pickedup at point A, but also at point B. The replica 8 of the transmissionline is, therefore, connected via resistors R3 and R9 or R5 and R6 bothon point A and on point B. This circuit arrangement is especiallysuitable for using a so-called “synthesized impedance”, in order to feedthe pure transmitted signal into the replica. Seen from the transformer13 in the direction of the line driver 1, the input impedance is definedby the serial circuit of the protective resistors R1 and R2 multipliedby the transmission ratio of the transformer 13. A reduction of theresistors R1 and R2 can be actively simulated by the line driver 1, as aresult of its output voltage being continually measured in order toimpress a corresponding current. This method of operation is describedas “synthesized impedance” and results in improved efficiency.

[0034] In general, the influence of the scatter inductance of thetransformer 13 can be simulated by parallel connection of capacitors tothose resistors, which emulate the protective resistors R1 and R2 and/orby corresponding dimensioning of the replica 8 of the transmission line.In the case of the circuit arrangement shown in FIG. 2, for this reason,capacitors C3-C6 are connected in parallel to the resistors R3-R6 inorder to emulate the influence of the scatter inductance of thetransformer 13.

[0035] Preferably, for emulating the main inductance of the transformer13, the parallel connection of two lowpasses is used, whereby the onelowpass is a lowpass of first order and the other lowpass a lowpass ofsecond order. The output voltages of the two lowpasses are weighted withfactors c1 or c2, whereby the ratio: c2=1−c1 preferably applies.

[0036] In the case of the circuit shown in FIG. 2, the weighting devices5-7, 11, 12 and 18 shown in FIG. 1 are implemented by correspondingresistor circuits. The adder 2 is implemented in the form of anamplifier circuit with variable regenerative resistors.

[0037] Simplification of the circuit is achieved by using a transformer13 with a so-called “sense winding”. This concerns an additionalwinding, which the transformer 13 has on its side facing the line driver1. In this case, the scatter inductance of the transformer 13 does nothave to be taken into consideration, whereby the overall signal is notpicked up at point B, but directly on the “sense winding” of thetransformer 13.

[0038] In the case of wired communication systems of high data rate, forexample, the communication system Gigabit Ethernet 1000Base-T, insteadof a line driver working as voltage source a so-called “current mode”line driver, that is to say a line driver working as power source, isfrequently used. In this case, modification of the embodiments shown inFIG. 1 and FIG. 2 as shown in FIG. 3 is recommended.

[0039] As shown in FIG. 3, a replica 19 of the “current mode” linedriver 1 is used. The line driver 1 is connected to the transformer 13,which represents the beginning of the transmission line 17. A resistorR7, which works as line terminal resistor, is connected in parallel withthe transformer 13. This line terminal resistor R7 can also possibly beactively emulated by the line driver 1 and then is no longer needed as acomponent part.

[0040] For emulating the line terminal resistor R7 (if present), aresistor R8 is provided, which is connected in parallel to the replica 8of the transmission line 17 and the replica 19 of the line driver 1. Theresistor R8 can also be integrated into the replica 8 of thetransmission line 17 and then is no longer present as a separatecomponent part.

[0041] The replica 19 of the line driver 1 emulates the behaviour of theline driver 1 as exactly as possible, whereby, if necessary, scaledemulation is also possible and receives the same transmitted data as theline driver 1. In this way, a signal is supplied into the parallelconnection by the replica 19 of the line driver 1 from the resistor R8and the replica 8 of the transmission line 17 which is identical to thetransmitted signal of the line driver 1. The voltage on point Dcorresponds in first approximation to the echo voltage on point A(=point B), whereby the behaviour of the transformer 13 as well as thebridge taps 14 are not yet taken into consideration. As in the case ofthe embodiments shown in FIG. 1 and FIG. 2, at least one lowpass 3, 4 isprovided for considering the behaviour of the transformer 13 and atleast one bandpass 9, 10 is provided for considering the bridge taps 14,to which, in each case, the voltage lying on point D is supplied asinput signal.

[0042] The components of the circuit arrangement shown in FIG. 3, whichare already shown in FIG. 1 or FIG. 2, correspond to the componentsshown in FIG. 1 and FIG. 2, so that, in this regard as well as regardsthe operating method of the circuit arrangement shown in FIG. 3,reference can be made to the above explanations regarding FIG. 1 andFIG. 2.

1. Circuit arrangement for the analogue suppression of echoes for thereceived signal of a communication device connected via a transformer toa transmission line, with first circuit means (8) for emulating thebehaviour of the transmission line (17), with second circuit means (3,4) for emulating the behaviour of the transformer (13), and with thirdcircuit means (2) for correcting a signal picked up on the transformer(13), which comprises a received signal portion and a transmitted signalportion of the communication device, by a signal of the first circuitmeans (8) and a signal of the second circuit means (3, 4), in order toobtain the received signal portion with suppressed transmitted signalportion, characterised in that the second circuit means comprise atleast one lowpass (3, 9), which is set up for emulating the behaviour ofthe transformer (13) and to which a signal is supplied as input signal,which corresponds to the transmitted signal to be transmitted by thecommunication device over the transmission line (17).
 2. Circuitarrangement according to claim 1, characterised in that thecommunication device comprises a line driver (1) and passive componentparts (R1, R2) connected between the line driver (1) and the transformer(13), whereby the first circuit means (8) is connected between the linedriver (1) and the passive component parts (R1, R2).
 3. Circuitarrangement according to claim 2, characterised in that the signal to becorrected, which comprises the received signal portion and thetransmitted signal portion, is picked up between the passive componentparts (R1, R2) and the transformer (13) and supplied to the thirdcircuit means (2).
 4. Circuit arrangement according to claim 1 or 2,characterised in that the signal to be corrected, which comprises thereceived signal portion and the transmitted signal portion, is picked upon an additional winding of the transformer (13) and supplied to thethird circuit means (2).
 5. Circuit arrangement according to any ofclaims 2-4, characterised in that the first circuit means comprise afirst circuit section (R3, R4), which is set up for emulating thebehaviour of the passive component parts (R1, R2) and a second circuitsection (8), which is set up for emulating the behaviour of thetransmission line (17).
 6. Circuit arrangement according to claim 5,characterised in that the first circuit section comprises resistors (R3,R4), which are set up for emulating the behaviour of the passivecomponent parts designed as protective resistors (R1, R2).
 7. Circuitarrangement according to claim 5 or 6, characterised in that the secondcircuit section (8) comprises a passive network, which is set up foremulating the behaviour of the transmission line (17).
 8. Circuitarrangement according to claim 7, characterised in that second circuitsection (8) comprises a passive RC network, which is set up foremulating the behaviour of the transmission line (17).
 9. Circuitarrangement according to any of claims 5-8, characterised in that thetransmitted signal of the line driver (1) or a signal picked up betweenthe first circuit section (R3, R4) and the second circuit section (8) ofthe first circuit means is supplied to the at least one lowpass (3, 4)of the second circuit means as input signal.
 10. Circuit arrangementaccording to any of claims 5-9, characterised in that the signal to becorrected, which comprises the received signal portion and thetransmitted signal portion, the output signal of the at least onelowpass (3, 4) of the second circuit device and the signal picked upbetween the first circuit section (R3, R4) and the second circuitsection (8) of the second circuit means are supplied to the thirdcircuit means (2), whereby the third circuit means (2) are designed insuch a way that it adds to the signal to be corrected, which comprisesthe received signal portion and the transmitted signal portion, theoutput signal of the at least one lowpass (3, 4) of the second circuitmeans and subtracts from this the signal picked up between the firstcircuit section (R3, R4) and the second circuit section (8) of the firstcircuit means, in order to obtain the received signal portion withsuppressed transmitted signal portion.
 11. Circuit arrangement accordingto claim 6 and any of the above claims, characterised in that in eachcase a capacitor (C3, C4) is connected in parallel to the resistors (R3,R4) of the first circuit section of the first circuit means.
 12. Circuitarrangement according to any of claims 5-11, characterised in that thefirst circuit section (R3-R6, C3-C6) is connected both between the linedriver (1) and the passive component parts (R1, R2) and between thepassive component parts (R1, R2) and the transmitter (13).
 13. Circuitarrangement according to any of the above claims, characterised in thatthe signal to be corrected, which comprises the received signal portionand the transmitted signal portion, is supplied to the third circuitmeans (2) via a weighting device (18) with a variable weighting factor(a).
 14. Circuit arrangement according to any of the above claims,characterised in that the signal supplied by the first circuit means (8)to the third circuit means (2) is supplied via a weighting device (7)with a variable weighting factor (7).
 15. Circuit arrangement accordingto any of the above claims, characterised in that the at least oneoutput signal of the at least one lowpass (3, 4) of the second circuitmeans is supplied via a weighting device (5, 6) with a variableweighting factor (c1, cn) to the third circuit means (2).
 16. Circuitarrangement according to any of the above claims, characterised in thatthe second circuit means comprise at least two parallel connectedlowpasses (3, 4), which are set up for emulating the behaviour of thetransmitter (13) and the output signals of which are supplied in eachcase to the third circuit means (2) for adding to the signal to becorrected, which comprises the received signal portion and thetransmitted signal portion, and that the one lowpass (3) is a lowpass offirst order and the other lowpass (4) a lowpass of second order. 17.Circuit arrangement according to claim 15 and claim 16, characterised inthat for the weighting factors of the weighting devices (5, 6) assignedto the two lowpasses (3, 4) the equation c2=1−c1 applies, whereby c1 isthe weighting factor assigned to the one lowpass (3) and c2 theweighting factor assigned to the other lowpass (4).
 18. Circuitarrangement according to any of the above claims, characterised in thatat least one parameter, in particular the limit frequency of the atleast one lowpass (3, 4) of the second circuit means is variable. 19.Circuit arrangement according to any of the above claims, characterisedin that fourth circuit means (9, 10), which are set up for emulating thebehaviour of stub cables (14) connected to the transmission line (17),are provided, whereby a signal of the fourth circuit means (9, 10) issupplied to the third circuit means (2) for correcting the signal, whichcomprises the received signal portion and the transmitted signalportion.
 20. Circuit arrangement according to claim 5 and claim 19,characterised in that the fourth circuit means comprises at least onebandpass filter (9, 10), to which as input signal the transmitted signalof the line driver (1) or a signal picked up between the first circuitsection (R3, R4) and the second circuit section (8) of the first circuitmeans, is supplied.
 21. Circuit arrangement according to claim 20,characterised in that the fourth circuit means comprise several parallelconnected bandpass filters (9, 10).
 22. Circuit arrangement according toclaim 20 or 21, characterised in that the output signal of the at leastone bandpass filter (9, 10) is supplied via a weighting device (11, 12)with a variable weighting factor (b1, bn) to the third circuit means(2).
 23. Circuit arrangement according to any of claims 20-22,characterised in that the output signal of the at least one bandpassfilter (9, 10) is supplied to the third circuit means (2) forsubtracting the signal to be corrected, which comprises the receivedsignal portion and the transmitted signal portion.
 24. Circuitarrangement according to any of claims 20-23, characterised in that atleast one parameter, in particular the quality, limit frequency and/oramplification, of the at least one bandpass filter (9, 10) of the fourthcircuit means is variable.
 25. Circuit arrangement according to any ofclaims 19-24, characterised in that the second circuit device (3, 4),the third circuit device (2) and the fourth circuit device (9, 10) areprovided on a common chip, whereas the first circuit device (8) isprovided outside the chip.
 26. Circuit arrangement according to any ofclaims 1 or 13-25, characterised in that the communication devicecomprises a line driver (1) for producing the transmitted signal, andthat further circuit means (19), which are set up for emulating thebehaviour of the line driver (1), are provided, whereby a signalproduced by the further circuit means (19) and corresponding to thetransmitted signal of the line driver (1) is supplied to the at leastone lowpass (3, 4) of the second circuit means as input signal. 27.Circuit arrangement according to claim 26, characterised in that theline driver (1) is a line driver (1) working according to the principleof a power source.
 28. Circuit arrangement according to claim 26 or 27,characterised in that the further circuit means (19), which are set upfor emulating the behaviour of the line driver (1), are coupled with thefirst circuit means (8), which are set up for emulating the behaviour ofthe transmission line (17), and that a signal picked up between thefurther circuit means (19) and the first circuit means (8) is suppliedto the at least one lowpass (3, 4) of the second circuit means as inputsignal.
 29. Circuit arrangement according to any of claims 26-28,characterised in that the communication device comprises a passivecomponent part (R7) connected between the line driver (1) and thetransformer (13), whereby the signal to be corrected, which comprisesthe received signal portion and the transmitted signal portion, ispicked up between the line driver (1) and the passive component part(R7) and is supplied to the third circuit means (2).
 30. Circuitarrangement according to claim 29, characterised in that the firstcircuit means comprises a first circuit section (R8), which is set upfor emulating the behaviour of the passive component part (R7), and asecond circuit section (8) coupled with the first circuit section (R8)for emulating the behaviour of the transmission line (17), and that asignal picked up between the further circuit means (19) and the firstcircuit section (R8) of the first circuit means is supplied to the atleast one lowpass (3, 4) of the second circuit means.
 31. Circuitarrangement according to claim 30, characterised in that the signal tobe corrected which comprises the received signal portion and thetransmitted signal portion, the output signal of the at least onelowpass (3, 4) of the second circuit means and a signal picked upbetween the further circuit means (19) and the first circuit section(R8) of the first circuit means is supplied to the third circuit means(2), whereby the third circuit means (2) are configured in such a waythat they add to the signal to be corrected, which comprises thereceived signal portion and the transmitted signal portion, the outputsignal of the at least one lowpass (3, 4) of the second circuit meansand subtracts from this the signal picked up between the further circuitmeans (19) and the first circuit section (R8) of the first circuitmeans, in order to obtain the received signal portion with suppressedtransmitted signal portion.
 32. Circuit arrangement according to claim30 or 31, characterised in that the second circuit section (8) of thefirst circuit means comprises a passive network for emulating thebehaviour of the transmission line (17).
 33. Circuit arrangementaccording to claim 32, characterised in that the second circuit section(8) of the first circuit means comprises a passive RC network, which isset up for emulating the behaviour of the transmission line (17). 34.Circuit arrangement according to any of claims 30-33, characterised inthat the first circuit section of the first circuit means comprises aresistor (R8), which is set up for emulating the behaviour of thepassive component part designed as resistor (R7).
 35. Circuitarrangement according to any of claims 30-34, characterised in that thefirst circuit section (R8), which is set up for emulating the behaviourof the passive component part (R7), is integrated in the second circuitsection (8), which is set up for emulating the behaviour of thetransmission line (17).